In terms of big epic spectacle like Walter said, I'd go with the death of Ganishka and the subsequent surprise appearance by Skullknight. The entire sequence, including the battle and the brief flashback of Ganishka's life, and then the panels with Skully appearing and Zodd caught off guard with Femto in his God Hand form is flawlessly done.
Honestly though another top "cool" moment is when Guts dons the armor for the first time and attacks the force sent to destroy Flora. I mean, it's just the coolest moment of the series. Doesn't get much better than that.
Runner up - The entire sequence towards the end of the Sea God episodes where Guts has his showdown with the massive creature by entering it and fighting his way through it's innards. That was a unique challenge for Guts and not the type of thing you get to see in much these days. As a young child, I was terrified when Pinocchio rescues Gepetto from Monstro the whale. That thing gave me nightmares and it stuck with me. It was great to see Miura utilize that in Berserk. Certainly a cool scenario for me.
Seeing that surprise Skullknight appearance, Femto's counter and the whole sequence of events after completely melted my mind. The fact that a series 34 books in could still blow me away the way that sequence did was incredible. Especially as a series that already had a million amazing moments.
Guts' battle with Rosine. Specifically that moment when...
[Looks up and sees rockvoanjd's post]
Okay, I guess a close second would be the finale of Guts' fight with Mozgus. Guts is on the ropes, nothing he does can even scratch the maniac, he doesn't know what to do...then he notices something. A chink in Mozgus' armor. He moves forward, takes a savage beating, gets thrown face-first into the masonry. Mozgus gears up to finish him off, but Guts only smirks and taunts the zealot, ending with: "Your faith might be your weakness, but on the other hand, it's where you're most vulnerable." And that's when we see the cavity in Mozgus' chest, where he keeps the remains of the Scripture, is filled with bombs. BOOM! They explode, blasting a hole open in Mozgus' chest@! And as the inquisitor is rearing back in pain, Guts takes advantage of the opportunity and attacks, striking the wound again and again and again. And just before he impales the beast on his sword, he lets out this awesome line: "If you see God, you tell him this: Leave me the hell alone!" Oooh, gets me every time I read it!
What it leads into is also really cool as far as spectacle goes. Guts flings Mozgus off the tower before the latter uses his final moments to immolate them both, sending his body crashing down in front of the crowd of refugees, now nothing more than a burning corpse. As the people look on in stunned silence, unable to comprehend what just happened to the one they thought was their savior, a child says: "Mother...the angel died." The mother screams. And then...
Chapter 110, right after Guts starting fighting with the Elves of Misty Valley at the end of the chapter theres this epic image of Guts after he lit all the eggs of fire. Literally made that all my profile pics.
I will always enjoy the moment Griffith confronts The Queen of midland and the nobles from the base of the tower while it burns as he tells her off and gloats. It is one of Griffith's most calculating and coldest moments that he just gave zero fucks about her status as the most powerful woman in the kingdom being the Queen and engineered her demise regardless of that. When he takes down his hair and and looks up at the tower I was amazed.